Discovered: March 14, 2003
Updated: February 13, 2007 11:44:43 AM
Type: Virus
Systems Affected: Windows


W32.Tosep is an encrypted Win32 virus that infects Windows Portable Executable (PE) files that have .exe extensions and reside in the same folder as the virus.




The PE files are portable files across all the Microsoft 32-bit operating systems. The same PE-format executable can be executed on any version of Windows 95, 98, Me, NT, or 2000. Therefore, all the PE files are executable, but not all the executable files are portable.

A good example of a PE is a screen saver (.scr) file.


Antivirus Protection Dates

  • Initial Rapid Release version March 17, 2003
  • Latest Rapid Release version September 28, 2010 revision 054
  • Initial Daily Certified version March 17, 2003
  • Latest Daily Certified version September 28, 2010 revision 036
  • Initial Weekly Certified release date March 19, 2003

Click here for a more detailed description of Rapid Release and Daily Certified virus definitions.

Writeup By: Yana Liu

Discovered: March 14, 2003
Updated: February 13, 2007 11:44:43 AM
Type: Virus
Systems Affected: Windows


When an infected program is executed, W32.Tosep does the following:

  1. Assumes control of the system.
  2. Searches for the PE files that have .exe extensions and reside in the same folder as the virus.
  3. Infects each file only once. This virus looks for an "FU**" string at the 58H offset of the PE header to see whether the file has already been infected.

    If the file has not been infected, the virus appends its encrypted body to the files. The infected file will have an appended section named "CODE2". This section is 4,096 bytes in length. Then, the virus changes the entry point of the host to its viral body and adds its infection marker, the "FU**" string, to the PE header.
  4. Appends 1,419 bytes (all zeros) to the end of the file. The infected file increases by a total of 5,515 bytes in length.


Recommendations

Symantec Security Response encourages all users and administrators to adhere to the following basic security "best practices":

  • Use a firewall to block all incoming connections from the Internet to services that should not be publicly available. By default, you should deny all incoming connections and only allow services you explicitly want to offer to the outside world.
  • Enforce a password policy. Complex passwords make it difficult to crack password files on compromised computers. This helps to prevent or limit damage when a computer is compromised.
  • Ensure that programs and users of the computer use the lowest level of privileges necessary to complete a task. When prompted for a root or UAC password, ensure that the program asking for administration-level access is a legitimate application.
  • Disable AutoPlay to prevent the automatic launching of executable files on network and removable drives, and disconnect the drives when not required. If write access is not required, enable read-only mode if the option is available.
  • Turn off file sharing if not needed. If file sharing is required, use ACLs and password protection to limit access. Disable anonymous access to shared folders. Grant access only to user accounts with strong passwords to folders that must be shared.
  • Turn off and remove unnecessary services. By default, many operating systems install auxiliary services that are not critical. These services are avenues of attack. If they are removed, threats have less avenues of attack.
  • If a threat exploits one or more network services, disable, or block access to, those services until a patch is applied.
  • Always keep your patch levels up-to-date, especially on computers that host public services and are accessible through the firewall, such as HTTP, FTP, mail, and DNS services.
  • Configure your email server to block or remove email that contains file attachments that are commonly used to spread threats, such as .vbs, .bat, .exe, .pif and .scr files.
  • Isolate compromised computers quickly to prevent threats from spreading further. Perform a forensic analysis and restore the computers using trusted media.
  • Train employees not to open attachments unless they are expecting them. Also, do not execute software that is downloaded from the Internet unless it has been scanned for viruses. Simply visiting a compromised Web site can cause infection if certain browser vulnerabilities are not patched.
  • If Bluetooth is not required for mobile devices, it should be turned off. If you require its use, ensure that the device's visibility is set to "Hidden" so that it cannot be scanned by other Bluetooth devices. If device pairing must be used, ensure that all devices are set to "Unauthorized", requiring authorization for each connection request. Do not accept applications that are unsigned or sent from unknown sources.
  • For further information on the terms used in this document, please refer to the Security Response glossary.

Writeup By: Yana Liu

Discovered: March 14, 2003
Updated: February 13, 2007 11:44:43 AM
Type: Virus
Systems Affected: Windows


The following instructions pertain to all current and recent Symantec antivirus products, including the Symantec AntiVirus and Norton AntiVirus product lines.

  1. Update the virus definitions.
  2. Run a full system scan and delete all the files detected as W32.Tosep.

For specific details on each of these steps, read the following instructions.

1. Updating the virus definitions
Symantec Security Response fully tests all the virus definitions for quality assurance before they are posted to our servers. There are two ways to obtain the most recent virus definitions:
  • Running LiveUpdate, which is the easiest way to obtain virus definitions: These virus definitions are posted to the LiveUpdate servers once each week (usually on Wednesdays), unless there is a major virus outbreak. To determine whether definitions for this threat are available by LiveUpdate, refer to the Virus Definitions (LiveUpdate).
  • Downloading the definitions using the Intelligent Updater: The Intelligent Updater virus definitions are posted on U.S. business days (Monday through Friday). You should download the definitions from the Symantec Security Response Web site and manually install them. To determine whether definitions for this threat are available by the Intelligent Updater, refer to the Virus Definitions (Intelligent Updater).

    The Intelligent Updater virus definitions are available here. For detailed instructions on how to download and install the Intelligent Updater virus definitions from the Symantec Security Response Web site, click here.

2. Scanning for and deleting the infected files
  1. Start your Symantec antivirus program and make sure that it is configured to scan all the files.
  2. Run a full system scan.
  3. If any files are detected as infected with W32.Tosep, click Delete.
  4. Replace the infected files from known, clean backups or re-install them if necessary.


Writeup By: Yana Liu